1. Field of the Invention
The present invention relates to the connection of a hollow sucker rod with a first and a second torque shoulder, and connecting elements with an axis, which are used to selectively rotate a rotary pump located deep down hole in an oil well from a drive head located at the surface of the oil well. The present invention comprises individual elements referred to herein as a “Hollow Sucker Rod” with at least a first end having a female thread and a “Connecting Element” which may be a separate “Nipple Connecting Element” with a pair of male threads or an integral male thread on a second, upset end of a Hollow Sucker Rod. In order to further optimize the stress distribution between the elements, frustro-conical, non-symmetrical threads with a differential diametral taper and two torque shoulders are used. The primary shoulder is located on the rod end and the secondary shoulder is located on the rod base. The hollow sucker rod and connecting element are dimensioned to obtain high operation torque, good fatigue resistance, good resistance to over torque and a surprising resistance to storing reactive torque, which minimizes dangerous backspin when power to the sucker rod string is interrupted.
2. Description of the Related Art
Non-surging oil well extraction is normally achieved by means of pumping systems. The most common system uses an alternating pump located at the bottom of the well driven by a sucker rod string that connects the bottom of the well with the surface, where an alternating pumping machine to drive the string up and down is located. The sucker rods in the prior art, therefore, were designed originally to simply reciprocate up and down, and were are manufactured to API Specification 11B using solid steel bars with an upset end and a threaded end, each thread being of solid cylindrical section. The rods typically were connected one with the other by means of a cylindrical threaded coupling. More efficient pumping is performed when an oil extracting progressive cavity pump (PCP), or like rotary down hole pump is used. Among other advantages, PCP pumping of oil allows for higher oil extraction rates, reduced fatigue loads, reduction in wear on the inside of production tubing, and the ability to pump high viscosity and high solids component oils. PCP pumps are installed at the bottom of the well and driven from the surface by an electric motor connected to a speed-reducing gearbox by means of a string of torque transmitting rods. Traditionally standard API sucker rods are used to drive PCP pumps notwithstanding the fact that these rods have not been designed to transmit torsional loads. The transmission of torque by means of conventional sucker rod strings presents the following disadvantages, i) low torque transmitting capacity, ii) high backspin iii) low resistance to overtorque, iv) big stiffness differential between the connection and the rod body, all factors that enhance the possibility of fatigue failures. The reason for rupture on this type of conventional rod is failure due to fatigue in the junction zone of the head of the rod with the body of same due to the difference in structural rigidity between both parts—the body of the rod and the head of the rod.
For a given cross sectional area, torque transmission by a hollow rod with an annular cross section is more efficient than with a narrower, solid circular cross section. With the above mentioned concept in mind the prior art includes a hollow sucker rod that simply uses a standard API external cylindrical thread on a first end connector and an internal API thread on a second end connector, each connector being butt welded to a pipe body, which creates significant and abrupt change in section between the pipe body and each connection body. (See, for example, EP 0145154 and JP04315605). The problem of sucker rod string backspin, and details of a drive head at the surface of an oil well and a rotary pump deep down hole in an oil well operation, which is the specific field of invention being addressed herein, can be found in Mills (U.S. Pat. No. 5,551,510), which is incorporated herein by reference.
The present invention is both specific to unique problems faced by a Hollow Sucker Rod, and categorically is different from threaded Drill Pipe connections in the following:                1) Drill pipe connections do not have severe constraints on the external dimensions of the pipe body and on the connection size. A hollow sucker rod external diameter is restricted to the internal diameter of the tubing, and typically is 2⅞″ and 3½″.        2) The flow speed of fluid that is conducted in the annular space between a hollow sucker rod and the inside of the well tubing is very limited, unlike the situation for a drill pipe.        
Various thread and shoulder arrangements are discussed in the prior art with respect to joining together oil well drill pipe, well casing and tubing. See, for example, Pfeiffer et al. (U.S. Pat. No. 4,955,644); Carstenson (U.S. Pat. No. 5,895,079), Gandy (U.S. Pat. No. 5,906,400), Mithoff (U.S. Pat. No. 262,086), Blose (U.S. Pat. No. 4,600,225), Watts (U.S. Pat. Nos. 5,427,418; 4,813,717; 4,750,761), Shock et al. (U.S. Pat. No. 6,030,004), and Hardy et al. (U.S. Pat. No. 3,054,628). The Watts patents imply that a pre-1986 API standard for strings of casing and tubing was a straight thread, with a turned down collar and that his improvement comprised a flush joint tubular connection with both tapered threads and a shoulder torque. Watts also refer to API standards for tubing and casing where triangular and buttress threads can be used with a torque shoulder. The 1990 patent to Pfeiffer et al, and the 1996 patent to Carstensen et al, in contrast, refer to a more current API standard (truncated triangular thread, connection using a torque shoulder) for strings of casing and tubing that appears to involve frusto-conical threads and shoulders. Carstensen et al at col 7, line 9+ include a discussion about how a particular conical gradient and length of a thread defines stress distribution results. Likewise, Pfeiffer et al at col 2, line 51+ say their threads are tapered and according to “API standards” with their improvement essentially only having to do with transitional dimensions. Hence, the problem addressed by Pfeiffer is an assembly of drill pipe sections where it apparently was critical to use a compatible and standard non-differential thread according to API standards, and also with no incomplete threads and no torque shoulder specification. The main features of the Pfeiffer thread appear to be symmetrical, truncated triangle threads (between 4 and 6 threads per inch, 60° flank angle) and a thread height that is the same for the male and female thread (between 1.42 and 3.75 mm). Also, there is identical nominal taper on male and female ends (between 0.125 and 0.25). Shock et al. illustrate a particular tool joint for drill pipe where the unexpected advantage for drill pipe applications derives from tapered threads that significantly must be very coarse (3½ threads per inch) and have equal angle (75°) thread flanks and elliptical root surfaces.
Prior art connections for drillpipe, casing and tubing which employ some manner of a second torque shoulder are shown: Schock (U.S. Pat. No. 6,030,004); Hallez (U.S. Pat. No. 5,169,183); Hori (U.S. Pat. No. 5,549,336); Hall (U.S. Pat. No. 4,548,431); Olivier (U.S. Pat. No. 6,485,063B1); Blose (U.S. Pat. No. 4,192,533); and Stone (U.S. Pat. No. 1,932,427).
Table 1, below, the principal characteristics of such prior art connections are compared with a Hollow Sucker Rod with Second Torque Shoulder according to the present invention, and also compared to Hollow Sucker Rods with a single torque shoulder as illustrated by SIDERCA (U.S. Pat. No. 6,764,108).
TABLE 1Principal Characteristics of Hollow Sucker Rods and others Connections with Second Torque ShoulderThreadDiametral Taper in/Threadsin on DiameterThread heightThreadLoad and StabProductThread Shapeper inch(Angle)(mm)CompletenessFlank angle [°] (*1)Hollow RodNon symmetrical8DifferentialN: 1.016N: CompleteLF: 4with onetruncated trapezoidN: 0.0976 (2.79°)P: 1.016P: Complete andSF: 9torqueP: 0.1 (2.86°)Incompleteshoulder(U.S. Pat. No. 6764108)Hollow RodNon symmetrical6-8DifferentialN: 1.016N: CompleteLF: 4with twotruncated trapezoidN: 0.0976 (2.79°)P: 1.016P: Complete andSF: 9torqueP: 0.1 (2.86°)IncompleteshoulderConnections with two or three torque shouldersSchock Pat.Symmetrical3½Non DifferentialN: ≧2.54N: CompleteLF: 32.5/42.5(U.S. Pat. No. 6030004)truncated trapezoid(API-drill pipe)P: ≧2.54P: CompleteSF: 32.5/42.5Hallez Pat.Symmetrical6-8Non DifferentialNAN: CompleteNA(U.S. Pat. No. 5169183)truncated trapezoid3-13*N & P: 0.035 aMaybe similar toP: Complete0.105 (1 a 3°)APIHori Pat.Symmetrical4-6Non DifferentialN & P: 1.42-3.75N & P: CompleteLF & SF: 30(U.S. Pat. No. 5549336)truncated triangle(API-Drill(API-drill pipe)(API-drill pipe)(API-drill pipe)(API-drill(API-Drill pipe)pipe)pipe)Hall Pat.Symmetrical4-6Non DifferentialN & P: 1.42-3.75N & P: CompleteLF & SF: 30(U.S. Pat. No. 4548431)truncated triangle(API-Drill(API-drill pipe)(API-drill pipe)(API-drill pipe)(API-drill(API-Drill pipe)pipe)pipe)Olivier Pat.Non symmetricalNANon DifferentialN: h1N: Complete andLF: −15(U.S. Pat. No. 6485063B1)truncated trapezoidN & P: 0.33 (9.37°)P: h2IncompleteSF: 20h1 > h2P: Complete|SF| > |LF|h1 − h2 = 0.05 mmBlose Pat.Non symmetricalNANon DifferentialNAN: CompleteLF: −15(U.S. Pat. No. 4192533)truncated trapezoidP: CompleteSF: 30|SF| > |LF|Stone Pat.SymmetricalNANon DifferentialNAN: CompleteNA(U.S. Pat. No. 1932427)truncated trapezoidN & P: 0.083 (2.5°)P: Complete(Modified Acme)UnionTorqueNo ofInternalshoulderTorqueClearancePrincipalProductbore formangle [°]shoulder(mm) (*3)LoadsObservationsHollow RodConical &711st. TS: 0.4 aTorsion-For hollow sucker rodwith oneCylindrical1.1Tension-Patent granted in USA,torqueBendingFrance and Argentinashoulder(U.S. Pat. No. 6764108)Hollow RodConical &721st. TS: 0.4 aTorsion-For hollow sucker rodwith twoCylindrical2.5Tension-Present inventiontorque2nd. TS: 0.4 aBendingshoulder2.53Connections with two or three torque shouldersSchock Pat.Cylindrical02NATorsion-For drill pipe(U.S. Pat. No. 6030004)(Maybe 1st. TSTension-N: Stress relief Groove& 2nd. TS: 0)BendingThread: elliptical rootsurfacesHallez Pat.Cylindrical<2-62NATorsion-For drill pipe(U.S. Pat. No. 5169183)(Maybe 1st. TSTension-N: Discharge groove& 2nd. TS: 0)BendingThread: Triangular,Trapezoidal or roundHori Pat.Cylindrical02NA (MaybeTorsion-For drill pipe(U.S. Pat. No. 5549336)(API-drill1st. TS: 0)Tension-Interchangeable with APIpipe)2nd. TS: 0.1 aBendingdrill pipe0.5Hall Pat.Cylindrical021st. TS: c1Torsion-For drill pipe(U.S. Pat. No. 4548431)(API-drill2nd. TS: c2Tension-2nd torque shoulder waspipe)c1 ≦ c2Bendingonly made for over torqueN&P: Relief groovesOlivier Pat.Cylindrical02NATorsion-For drill string(U.S. Pat. No. 6485063B1)(Maybe 1st. TSTension-Thread: LF has S-Shape& 2nd. TS: 0)BendingTS: Curved SurfaceThread: Buttress, API,ACME, etc.Blose Pat.Cylindrical53NA—For tubing, casing,(U.S. Pat. No. 4192533)linepipe and drillpipeStone Pat.Cylindrical1st. TS:21st. TS: c1—For drillpipe and casing(U.S. Pat. No. 1932427)302nd. TS: c22nd. TS:c1 ≧ c2−40Nomenclature:N = NippleP = PipeC = CouplingNA = Not ApplicableLF = Load FlankSF = Stab FlankTS: Torque shoulder(*1) Angle defined from a perpendicular to the pipe axis.(*3) Clearance between torque shoulder surfaces of pipe and nipple after the hand-tightened of the connection;1st. TS: First torque shoulder or external torque shoulder;2nd. TS: Second torque shoulder or internal torque shoulder
TABLE 2Principal Characteristics of Hollow Sucker Rods and others Connections with only one Torque Shoulder (U.S. Pat. No. 6764108)ThreadDiametral Taper in/Threadsin on DiameterThread heightThreadLoad and StabProductThread Shapeper inch(Angle)(mm)CompletenessFlank angle [°] (*1)Hollow RodNon symmetrical8DifferentialN: 1.016N: CompleteLF: 4with one torquetruncatedN: 0.0976 (2.79°)P: 1.016P: Complete andSF: 9shouldertrapezoidP: 0.1 (2.86°)Incomplete(U.S. Pat. No. 6764108)Hollow RodNon symmetrical6-8DifferentialN: 1.016N: CompleteLF: 4with two torquetruncatedN: 0.0976 (2.79°)P: 1.016P: Complete andSF: 9shouldertrapezoidP: 0.1 (2.86°)IncompleteConnections with one Torque ShoulderPfeiffer Pat.Symmetrical4-6Non DifferentialN & P: 1.42-3.75N & P: CompleteLF & SF: 30truncated triangle(ApI-drill pipe)(API-drill pipe)(API-drill pipe)(API-drill(API-Drill pipe)pipe)Watts Pat.Symmetrical(API-DifferentialLess than APIN: CompleteLF: ≦15truncated triangleTubing)P: Complete and(API-Tubing)IncompleteDrill Pipe (API)Symmetrical4-6Non DifferentialN & P: 1.42-3.75N & P: CompleteLF & SF: 30truncated triangleN & P: 0.125-0.25Tubing API 8rSymmetrical10-6 (*2)Non Differential1.8C: CompleteLF & SF:: 30truncated triangleC & P: .0625P: Complete andIncompleteCasing API 8rSymmetrical8Non Differential1.8C: CompleteLF & SF:: 30truncated triangleC & P: .0625P: Complete andIncompleteCasing APINon symmetrical5Non Differential1.575C: CompleteLF: 3ButtresstruncatedC & P: .0625P: Complete andSF: 10trapezoidIncompleteCasing APISymmetrical6Non DifferentialC: 1.52C: CompleteLF: 6Extreme LinetruncatedC & P: .0625P: 1.35P: Complete andtrapezoidIncompleteUnionTorqueNo ofExternalInternal boreshoulderTorqueSurface ofProductformangle [°]shoulderConnectionPrincipal LoadsObservationsHollow RodConical &71FlushTorsion-Tension-For hollow suckerwith one torqueCylindricalBending.rodshoulderPatent granted in(U.S. Pat. No. 6764108)USA, France andArgentinaHollow RodConical &72FlushTorsion-Tension-For hollow suckerwith two torqueCylindricalBendingrodshoulderPresent inventionConnections with one Torque ShoulderPfeiffer Pat.CylindricalNA1Non FlushTorsion-Tension-For drillpipe(API-drillBending.pipe)Watts Pat.Cylindrical—1FlushTension-For tubingCompression-Internal Pressure-External PressureDrill Pipe (API)Cylindrical01FlushTorsion-Tension-For drillpipeBending.Tubing API 8rCylindricalNA1Non FlushTension-For tubingCompression-Internal Pressure-External PressureCasing API 8rCylindricalNA1Non FlushTension-For casingCompression-Internal Pressure-External PressureCasing APICylindricalNA1Non FlushTension-For casingButtressCompression-Internal Pressure-External PressureCasing APICylindrical01Non FlushTension-For casingExtreme LineCompression-Internal Pressure-External PressureNomenclature:N = NippleP = PipeC = CouplingNA = Not ApplicableLF = Load FlankSF = Stab FlankTS: Torque shoulder(*1) Angle defined from a perpendicular to the pipe axis.(*2) Non Upset Tubing 1.66″ to 3.5″: 10 threads per inch., 4″ and 4.5″: 8 threads per inch. Upset Tubing 1.66″ and 1.9″: 10 threads per inch, 2.325″ to 4.5″: 8 threads per inch.
Table 2, above, illustrates the principal characteristics of a hollow sucker connection with one torque shoulder, as compared to a hollow sucker rod with one torque shoulder. Another version of a single torque shoulder, with a second engagement surface that acts as a seal but does not transmit torque, is illustrated herein at FIGS. 13 and 14.
However, the different problem of backspin inherent in the intermittent operation of a sucker rod string when driving a PCP pump is not apparently addressed in any of these references. The design of the invention was made with certain specific constraints and requirements in mind.
First, the minimum diameter of a tubing on the inside of which the Hollow Rods must operate corresponds to API 2⅞″ tubing (inner diameter=62 mm) and API 3½″ tubing (inner diameter=74.2 mm). The oil extraction flow rate must be up to 500 cubic meters per day, maximum oil flow speed must be 4 meters per second. The above-mentioned values strongly restrict the geometry of the rods under design. Second, to ensure a Hollow Sucker Rod with a high yield torque so that maximum torque is transmitted to the PCP pump without damage to the Hollow Sucker Rod string. Third, to minimize and distribute stresses in the threaded sections. This requirement is met by using a particular conical thread, differential taper, low thread height and a conical bore in the sections under the threads. Fourth, the Hollow Sucker Rod must have good fatigue resistance. Fifth, to ensure low backspin, and high resistance to axial loads. Sixth, ease of make up and break out (assembly of mating threaded parts) must be ensured, and is by a tapered thread. Seventh, to ensure high resistance to unscrewing of the Hollow Sucker Rod due to backspin, or the counter-rotation of a sucker rod string when driving motor stops running and the pump acts as a motor. Eighth, to ensure high resistance to jump out of the Hollow Sucker Rod string (Hollow Rod parting at the threaded sections) by means of adequate thread profile and reverse angle on the torque shoulder. Ninth, to minimize head loss of the fluids that occasionally can be pumped on the inside of the Hollow Sucker Rod through the added advantage of a conical bore on the nipple and the secondary torque shoulder. Tenth, to ensure connection sealabilty due to a sealing at both torque shoulders, and also due to diametrical interference at the threads. Eleventh, a thread profile designed so as to optimize pipe wall thickness usage. Twelfth, to eliminate use of the welds due to susceptibility of welds to fatigue damage, sulphide stress cracking damage and also the higher costs of manufacturing. Thirteenth, when a fluid flows through the interior of the rod with reasonable speed, it produces early wear of the nipple and rod in the area where they connect (overlap), hence, a seal was introduced by virtue of a secondary torque shoulder at each end of the nipple, which also ensures high resistance to an over torque of the connection. Fourteenth, to substantially increase the flow of fluid extracted, holes in the rod body were drilled to allow the fluid flowing through the interior of the rod.
A first object of the present invention is to provide an assembly of sucker pump rods and either separate threaded unions, or an integral union at the second end of each sucker rod, to activate PCP and like rotary type pumps, capable of transmitting greater torque than the solid pump rods described in the API 11 B Norm and also possessing good fatigue resistance, and improved resistance to over torque. Additionally, the present invention seeks to define a threaded union for hollow sucker rods that is significantly different from, and incompatible with, the standard for sucker rod assemblies as defined in the API 11 B Norm, yet still can easily be assembled. In fact the modified buttress thread is unique in that it is differential. For example, API Buttress Casing requires non-differential threads, with the taper for both a pipe and a coupling being 0.625 inches/inch of diameter. Likewise, API 8r casing and API 8r tubing both also require non-differential threads, with the taper for both a pipe and a coupling being 0.625 inches/inch of diameter. Still further, each of API Buttress Casing, API 8r casing and API 8r tubing do not employ any manner of torque shoulder, let alone first and second torque shoulder. For example, in Table 2 the connections show one torque shoulder.
A related object of the present invention is to provide an assembly of pump rods and unions with lesser tendency to uncoupling of the unions whenever “backspin” occurs, whether by accident or when intentionally provoked by the deactivation of the pump drive. The present invention surprisingly and significantly decreases the stored torsional energy in a sucker rod string. The stored energy in the string is inversely proportional to the diameter of the rod, and is directly proportional to the applied torque and the length of the string.
Another object of the invention is to provide for an assembly of sucker rods which are hollow and configured with a bore to permit passage of tools (sensors for control of the well) and/or allow interior circulation of fluids (injection of solvents and/or rust inhibitors).
The two torque shoulder embodiments disclosed herein have bigger yield torque than a hollow sucker rod with only one torque shoulder, as illustrated by U.S. Pat. No. 6,764,108.
The two torque shoulder, eighth and ninth embodiments disclosed herein have a yield torque of the connection that is up to 110 percent more than an otherwise corresponding hollow sucker rod with only one torque shoulder.
Still another object of the invention is to further optimize the stress distribution between the elements, by the combination of using frustro-conical, non-symmetrical threads with a differential diametral taper and two torque shoulders. The primary or first rod torque shoulder is located on rod end and the secondary or second rod torque shoulder is located on the rod base. The hollow sucker rod and connecting element are dimensioned to obtain high operation torque, good fatigue resistance, good resistance to over torque and a surprising resistance to storing reactive torque, which minimizes dangerous backspin when power to the sucker rod string is interrupted.